1. Field of the Invention
The present invention relates to an electric device including an actuator unit for performing printing by ejecting ink droplets onto a recording medium.
2. Description of Related Art
In an inkjet printhead used in an inkjet printer, as an example of an actuator unit used in an electric device, ink supplied from an ink tank is distributed to a plurality of pressure chambers, and ink droplets are ejected by application of pressure in the form of pulses selectively for the respective pressure chambers. As such an actuator unit, one comprising a laminate of a plurality of piezoelectric sheets of piezoelectric ceramic can be used.
JP-A-2003-311953 discloses an arrangement where a plurality of such actuator units are supported in an inkjet head unit. Each of the actuator units is constructed such that continuous flat piezoelectric sheets are interposed among a plurality of common electrodes extending across a plurality of pressure chambers while a plurality of individual electrodes, each comprising a main portion and an auxiliary portion at which voltage is applied from the outside, are disposed at positions corresponding to the respective pressure chambers. In the inkjet head unit, the individual electrodes are electrically connected to a flexible printed wiring board or a flexible printed circuit board (FPC). The FPC has a plurality of connection pads positionally corresponding to the auxiliary electrode portions, and each of the connection pads is bonded to a corresponding one of the auxiliary electrode portions with a solder or other conductive materials. A portion of the piezoelectric sheets, which is interposed among the individual and common electrodes and polarized in the direction of stacking of the piezoelectric sheets and common and individual electrodes, expands and contracts in the stacking direction by the piezoelectric longitudinal effect, when the electrical potential at the individual electrodes is differentiated from that of the common electrodes by application of drive voltage via the FPC. Accordingly, the inner volume of the corresponding pressure chamber is changed to eject an ink droplet from a nozzle communicated with the pressure chamber onto a recording medium.
Recently, to meet the demand for printing at a higher resolution and higher speed, the pressure chambers have come to be arrayed in a high density, decreasing a bonding area between each connection pad of the FPC and the auxiliary electrode portion. This leads to a contact between adjacent two solder bumps each connecting the pad and the auxiliary electrode portion, causing a short-circuit between the individual electrodes. The amount or volume of the solder bonding the pad and the auxiliary electrode portion may be reduced to prevent the solder bump from running off an edge of the bonding area in order to prevent this contact between the solder bumps. However, this now deteriorates the strength of the bond between the pad and the auxiliary electrode portion, causing an electrical disconnection due to separation of the pad from the auxiliary electrode portion.
Meanwhile, JP-A-4-33550 discloses an FPC of an inkjet printer where a terminal of each of conducting layers constituting a conductor pattern is disposed at a position to be opposed to a piezoelectric patch. The terminal has a portion protruding toward the piezoelectric patch. The FPC is pushed or pressed by a fixing member via an elastic member such that the protrusions of the respective terminals of the FPC are held in contact with the respectively corresponding piezoelectric patches. Although it is possible to electrically connect an FPC with a piezoelectric patch without using solder in this way, such an arrangement requires additional members to press the FPC against the piezoelectric patch, namely, the elastic member, fixing member, etc., making the structure of the head unit or apparatus using the head unit relatively complex. Further, the arrangement where the protrusions of the terminals of the FPC are kept pressed onto the piezoelectric patches by the pressing force of the elastic member and fixing member leads to damage of the piezoelectric patch having a low mechanical strength.